1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device that achieves improvement of structure of gate parts in a stack-gate configuration including a floating gate electrode and a control gate electrode.
2. Description of the Related Art
Memory cells for use in nonvolatile semiconductor memory devices have a stack-gate structure that is composed of a tunnel insulating film, a floating gate electrode, an inter-electrode insulating film, and a control gate electrode, which are provided on a semiconductor substrate, one upon another in the order mentioned. Write and erase in and from the memory cell is performed by applying a charge into, and taking it charge from, the gap between the tunnel insulating film and the floating gate electrode. Applying the charge taking it are achieved by applying a high voltage between the control gate electrode and the substrate, thereby causing an Fowler-Nordheim (FN) tunnel current to flow.
The threshold voltage is controlled in accordance with whether a charge exists in the floating gate or not, thereby to store data. It is therefore important that the inter-electrode insulating film should have high permittivity, that the coupling ratio should not decrease and that the threshold voltage should not change after data has been written or erased.
Hitherto, SiO2/SiN/SiO2 film (hereinafter referred to as ONO film), which has higher permittivity than silicon oxide film, has been used as inter-electrode insulating film. Recently, it is proposed that material having higher permittivity than ONO film (hereinafter referred to as high-permittivity material) be used. In particular, a high-permittivity oxide film, such as hafnium oxide film or aluminum oxide film, has high thermal stability and very compatible with the steps of manufacturing semiconductor elements. It is therefore expected that a high-permittivity oxide film, such as hafnium oxide film, aluminum oxide film, or film of a mixture of these oxides, will be used as next-generation inter-electrode insulating film. (See Jpn. Pat. Appln. KOKAI Publication 2003-7861.)
If such a high-permittivity insulating film is used, however, an electric charge will be trapped in the inter-electrode insulating film when an electric field is applied. Further, the charge thus trapped will be de-trapped. Inevitably, the threshold voltage will change with time. Consequently, data cannot be fully written into, erased from, or held in, the memory cell as is desired.
If the inter-electrode insulating film is a high-permittivity film, the change of the threshold voltage, which occurs when data is written into, erased from or held in, the memory cell, cannot fall within a range demanded in view of the device specification. Moreover, if a high-permittivity insulating film is used as tunnel-insulating film, as is proposed, similar problems will arise.